Field of the Disclosure
The present invention relates to an electron microscope, and particularly to a multifunctional ultrafast electron gun of a transmission electron microscope.
Description of the Related Art
A transmission Electron Microscope (TEM) is designed to project accelerated and focused electron beam to a very thin sample such that electrons hit atoms in the sample and consequently moving direction of electrons is changed, thereby generating a three-dimensional scatter of electrons. A scattering angle of electrons is dependent upon density and thickness of the sample. Upon being scattered, an image with different shadings at different portions is Obtained. The resultant image is then displayed on an imaging device (such as, screen, photographic film or photo-coupler assembly) after being amplified and focused.
From the entire development process of a transmission electron microscope, discovery of electron and its wave-particle duality become a basis of a two-dimensional imaging of transmission electron microscopy. Use of spherical aberration corrector has remarkably improved spatial resolution of the transmission electron microscope, which currently reaches 1 Å or less, and even exceeds 0.5 Å. This resolution had met most of the requirements for analyzing structure of material, because, in most circumstances, the distance between neighboring atoms is more than 1 Å. Thus, researchers tend to enhance resolutions of a transmission electron microscope with respect to other dimensions. For example, energy resolution of electron energy loss spectroscopy has been raised up to 0.1 eV by using energy monochromator.
With development of material, chemistry and condensed matter physics, in study of the dynamic process of material, a strong desire had been raised on another dimension-time resolution for a transmission electron microscope, i.e., requiring to detect a transient state in a sufficient short time (for example, within several nanoseconds or even femtoseconds).
Compared with the conventional TEM, a time-resolved transmission electron microscope has advantages that a variety of experimental techniques can be integrated so as to study a dynamical behavior of matter with a high spatial resolution, energy resolution and time resolution. Observation on transient states and transient state behaviors (such as, chemical reactions, structural deformation or phase changes, etc.) is the key to understand many basic behaviors in chemistry, biology, physics and materials science. In chemistry field, key point is how to comprehend kinetics processes and reaction mechanism of a chemical reaction, such as, changes of different activity positions of catalyst, which is the central problem of femtosecond chemistry. In physics and materials science, the study of dynamical process of various phase transition behaviors, such as the structural phase transition, the metal-insulator transition and competition of various phases under external fields, forms a basis of understanding the mechanisms of various physical properties of material. In biology and medicine, studying structures of different biological molecules, such as cells, proteins, etc., facilitates understanding of the primary function of them in the living body, which can greatly promote development of modern medicine and biology.
Critical technology of time-resolved transmission electron microscopy is to create a non-continuous emission and ultrashort electron-pulse electron source. It is possible to achieve a high time-resolution by controlling the electron. source by a femtosecond laser system so as to limit an electron pulse width to a femtosecond scale. There are two laboratories in the United States, where a nanosecond and femtosecond time-resolved transmission electron microscopy is achieved by reforming the optical system of TEM, both modifying the optical system of commercially available transmission electron microscope to achieve a TEM techniques with a time-resolved characteristics.
FIG. 1 is a schematic structural view of a time-resolved transmission electron microscope in prior art modified by B.W Reed Research Group. Referring to FIG. 1, the shown time-resolved transmission electron microscope is modified by adding a section of weak magnetic lens and an electron brass drift section over an illumination system, i.e., a condenser lens system, to a conventional transmission electron microscope. The electron brass drift section includes a laser incoming window for introducing laser light to be incident to the cathode and a laser reflective mirror for reflecting the incoming laser light to the cathode of the electron gun of the TEM to excite photoelectrons. The laser reflective mirror is treated to have an aperture for electron beam being passed therethrough, through which the electrons accelerated by the anode are passed to enter the condenser lens system of the TEM. The weak magnetic lens is provided to converge electrons with rather big scattering angles so as to obtain a bigger electron beam flow.
However, in the process of implementing the present invention, the applicant has found that, in the above-mentioned time-resolved transmission electron microscope, a near distance of 30 cm between the anode and the condenser lens system was added due to adding the electron brass drift section, which resulted the electrons exiting from the anode to pass through a long drift distance and became to travel in an increased divergence angle, and in turn the number of electrons entering the condenser lens system became less and dose of electrons turned lower. Even a weak magnetic lens was added to increase the number of electrons, the electron dose was less than that in conventional TEM. Furthermore, the electron coherence became poor due to the increased divergence angle of the collected electrons.